Thin flat display panels, such as plasma displays (PDPs) and liquid crystal displays (LCDs), have been widely used, and organic electroluminescent (EL) panels using organic EL elements are appearing in the market place.
Organic elements use organic materials as, for example, a light emission material, and the lifetime of the organic materials would be shortened if they contain moisture. To prevent this, a sealing substrate is arranged to face an EL substrate, where EL elements are arrayed in matrix, at a predetermined distance from the EL substrate. The periphery of the substrates is sealed with a seal along the periphery of the substrates in an airtight manner through a resin sealing material to prevent entry of moisture. In addition, a desiccant is encapsulated in the inner space sealed by the sealing substrate to remove moisture.
Typically, a sealing material is made of, for example, an epoxy-based resin which is cured by ultraviolet irradiation. To provide a more airtight seal, however, use of a low melting point glass, such as solder glass, has also been proposed (see, for example, Japanese Patent Laid-open Publication No. 2001-319775).
Each pixel of the organic EL panel is formed by an EL element and other components including a switching TFT for controlling current flow to the EL element, a current drive TFT, etc. These pixels are arrayed in matrix on the element substrate to display images on the entire panel by controlling the display of each pixel.
Organic EL panels include bottom-emission type panels and top-emission type panels, classified according to the direction of light emission. In bottom-emission type panels, light is emitted from a light emitting layer of the organic EL element through the element substrate, while top-emission type panels emit light through the sealing substrate. Top-emission type panels are disclosed in, for example, Japanese Patent Laid-open Publication No. 2002-299044. Advantageously, a larger aperture ratio and higher luminance can be achieved in top-emission type panels, because light is emitted from the light emitting layer without being interrupted by TFTs of each pixel.
A color filter system has been adapted in organic EL panels, which combines one (white) type of light emitting layer with color filters in order to display color images. In the top-emission configuration, color filters are formed on the sealing substrate.
Japanese Patent Laid-open Publication No. 2001-319775 discloses the use of low melting point glass for bonding, wherein the glass is sandwiched between glass substrates (i.e., the element substrate and the sealing substrate) and heat is applied to the glass to weld the substrates together. The low melting point glass is disposed at a desired location by applying a glass paste made of powder of the low melting point glass mixed with a resin binder. Once the glass paste is applied on the substrate, a thermal processing is conducted typically at 450° C. or more for more than several ten minutes in order to remove the solvent from the glass paste and solidify the glass paste.
The color filters used for the organic EL panels are made of organic resins, so that the filters are susceptible to heat, usually having a heat resistance of not more than 200° C.
It is therefore impossible to conduct a thermal processing to the substrate after the color filters are formed on the substrate and the glass paste is applied thereon. Conversely, if the glass paste is applied on the substrate before the color filters are formed, the glass paste forms a convex portion on the surface of the substrate. When such a convex portion exists on the surface where the color filter is to be formed, the color filters cannot be disposed accurately at a predetermined position, because the filters are formed by patterning a resist or the like. In the meantime, a black matrix may be disposed between pixels to prevent mingling of light from each pixel or, alternatively, a phase plate and/or a polarizer are provided to prevent glaring touch or reflection of the display. In the top-emission type panels, the black matrix, the phase plate, and/or the polarizer, etc. are also formed on the side of the sealing substrate and are usually made of resin. Therefore, the black matrix has the same characteristic as the above-mentioned color filters.